D3 is attempting something relatively new with the skill-rune system, and given blizzard's immense funding and development potential, I see no reason why they should limit their creativity to the skill system alone.

Here is what I propose...
Elements will now interact not only with each other, but also with themselves.

Considering that each class, some more so than others of course, will have access to elemental effects for their skills/weapons this system can be worked into itemization and socketing also.

For example, if a weapon or spell with a temporary freeze effect is followed by a lightning effect while the monster is still incapacitated, the electric energy will resonate through the frozen ice and cause it to shatter, possibly with an AoE damaging effect. On the other hand, if the ice spell were followed with a simple direct damage fire spell, the duration of the freeze would be countered and the monster would resume attack.

If a monster that is infected with a DoT Poison effect is ignited with a Fire spell, the massive increase in temperature will cause the poison to vaporize within the body and veins, then travel briefly as a deadly AoE Poison gas.

If a group of monsters are struck with a Ice spell that slows (not freezes) their movement/attack, then to strike one with a Lightning effect would cause the lightning to jump to any monsters in proximity.

Fire + Lightning as direct damage attacks would cause the lightning and fire to magnify and dramatically increase the targets temperature, essentially incinerating the target by multiplying each others damage.

Ice effect added to a Poison DoT would cause the blood to thicken/freeze and increase the duration of the poison effect considerably.

Progressive/stackable Llightning strikes to one enemy will cause seizure/stun effects with increasing chance of "instant death" up to a certain percentage.

Progressive Fire attacks will shorten burn time with each strike, but not shorten burn damage, so 100 burn dmg over 10 seconds will be 100 dmg over 9/8/7/etc.. for each consecutive attack.

Ice effect on a 1hand weapon for example will continuosly decrease movement/attack speed for each hit landed, up to a limit based on level/skills/etc... eventually leading to a full freeze effect if situation allows it.

This of course is barely scratching the surface if we consider possible effects from 3 different elements combined (freeze with Ice, melt with Fire, shock with Lightning) or from 1 element to another and back to the original. This should only be designed as a small "game within a game" that does not overpower and certainly not overshadow the full effects of spells and skills, but only to create more interest for melee/ranged generic attacks which do not consume mana.

The system could of course be inverted as well, allowing the player to imbue/infuse their armor with elemental effects in response to enemy elements.

For example, armor enhanced with fire can burn enemies at close range and will cause you to regain small amounts of Health or Mana for every fire attack you are hit with, but it will increase damage you take from all lightning attacks.

If anyone has any ideas of possible effects please suggest them, I am not entirely confident in this system as of yet, but perhaps if enough unique and interesting variations and combinationss are collected there may be some substance to this whole thing.